This application claims Paris convention priority of European Patent Application No. 99 103 142.8 filed on Feb. 18, 1999, the complete disclosure of which is hereby incorporated by reference.
The invention refers to a method for performing polarization transfer in a nuclear magnetic resonance (=NMR) experiment with spin systems of large molecules, especially biological macromolecules in solution, comprising at least two kinds of spin xc2xd nuclei I and S being coupled to each other, the spin system being subjected to a homogeneous magnetic field B0, being irradiated by a sequence of radio frequency (=rf) pulses comprising a first 90xc2x0 pulse exciting the spins of the nuclei I and after a delay time a further 90xc2x0 pulse exciting the spins of the nuclei S.
Such a method is used in the INEPT-type experiments published by Morris and Freeman, J. Am. Chem. Soc. 101, (1979) p. 760-762, describing magnetisation transfer via spin-spin couplings.
For the study of large biological macromolecules, the INEPT sequence is nowadays widely used as transfer element for heteronuclear NMR experiments. However, for molecular weights beyond 100000, the transfer time becomes a limiting factor and the INEPT sequence will fail to yield good results.
It is therefore an object of the present invention, to improve the INEPT method and provide a novel polarization transfer element which can be used as a xe2x80x9cbuilding blockxe2x80x9d for a great variety of complex NMR experiments including macromolecules with molecular weights far beyond 100000 and yielding higher sensitivity in comparison with methods according to the state of the art.
The objects of the present invention are achieved in that the sequence of rf pulses is chosen such that there is no inversion pulse acting on the spins of the nuclei S during a time period T between the first 90xc2x0 pulse exciting the spins of the nuclei I and either the further 90xc2x0 pulse exciting the spins of the nuclei S or a second 90xc2x0 pulse acting on the spins of the nuclei I, and that the length of the time period T is chosen such that
d/dT[{square root over (sin h+L (RCT+L )2+L +sin(xcfx80JIST+L )2+L )} exp(xe2x88x92RIT)]
is minimized, where
RC is the transverse cross-correlation-relaxation rate of nuclei I,
RI is the total transverse relaxation rate of nuclei I and JIS is the scalar coupling constant between nuclei I and S.
Thus, the main features of the INEPT method transferring magnetization via spin-spin couplings can be combined with the advantages of cross-correlated relaxation-induced polarization transfer. This combination can be mainly achieved by the omission of the refocussing and inversion pulses during the time period T. This is, at the first glance, surprising because of the usual idea that those pulses are in any case necessary for the detection of magnetization after the application of the rf pulse sequence since during the time period T the magnetization components disperse and are hence attenuated to a large degree. However, the method according to the present invention has turned out to work anyway with large molecules, since there seems to be still enough magnetization despite the omission of a refocussing mechanism, because the inventional rf sequence more than compensates the mentioned signal losses.
In a preferred variant of the inventional method, a magnetic field gradient G1 is applied within the time period T, allowing to eliminate artefacts.
In another preferred variant of the invention a 180xc2x0 pulse acting on the nuclei I is irradiated in the middle of the time period T, thus refocussing the magnetization due to the chemical shift and selecting only the magnetization transfer by cross-correlated relaxation.
An improved version of this variant is characterized in that a magnetic field gradient G1 is applied within the time period T/2 before the 180xc2x0 pulse and another magnetic field gradient G1 is applied within a time period T/2 after the 180xc2x0 pulse, whereby artefacts can be efficiently eliminated.
In order to obtain single quantum coherence, in another variant of the inventional method the further 90xc2x0 pulse exciting the spins of nuclei S is irradiated at the same time as the second 90xc2x0 pulse acting on the spins of the nuclei I.
Alternatively, the inventional method can be performed such that the further 90xc2x0 pulse exciting the spins of nuclei S is following up the second 90xc2x0 pulse acting on the spins of the nuclei I after a time delay.
In an improved version of this variant, a magnetic field gradient G2 being applied within the delay time between the second 90xc2x0 pulse acting on the spins of the nuclei I and the further 90xc2x0 pulse exciting the spins of nuclei S. This leads to the elimination of magnetization components at the end of the sequence, which are of no interest in the experiment.
In another alternative variant of the inventional method, the further 90xc2x0 pulse exciting the spins of nuclei S is irradiated after the time period T following up the first 90xc2x0 pulse exciting the spins of nuclei I, and the second 90xc2x0 pulse acting on the spins of the nuclei I is being omitted, thus allowing to obtain zero and double quantum coherence.
Another preferred variant of the method according to the present invention is characterized in that the sequence of rf pulses comprises a 180xc2x0 pulse acting on the nuclei I irradiated at the same time as the further 90xc2x0 pulse exciting the spins of nuclei S after the time period T following up the first 90xc2x0 pulse exciting the spins of nuclei I and that the second 90xc2x0 pulse acting on the spins of nuclei I is irradiated after a second time period T following up the 180xc2x0 pulse acting on the nuclei I. This allows refocussing the evolution of the magnetization due to the chemical shift.
In an improved version of this variant, a magnetic field gradient G1 is applied within the first time period T and another magnetic field gradient G1 is applied within the second time period T, thereby eliminating artefacts.
In a preferred variant, at the beginning of the experiment before the irradiation of the first 90xc2x0 pulse exciting the spins of nuclei I a 90xc2x0 pulse acting on the spins of the nuclei S is irradiated followed up by the application of a magnetic field the S-magnetization is excluded from the further evolution of spins in the system under observation.
It can be of advantage to the inventional method, when the sequence of rf pulses comprises a part adapted to suppress NMR signals of a solvent.
Also may it be of advantage, when the sequence of rf pulses comprises a part adapted to maintain the magnetization of a solvent along the homogeneous magnetic field B0.
In common multidimensional NMR experiments for studies of biological macromolecules in solution, magnetization transfers via spin-spin couplings (INEPT) are key elements of the pulse schemes. For molecular weights beyond 100""000, transverse relaxation during the transfer time may become a limiting factor. This invention presents a novel transfer technique for work with big molecules, called CRINEPT, which largely eliminates the size limitation of INEPT transfers with the use of cross-correlated relaxation-induced polarization transfer. The rate of polarization transfer by cross-correlated relaxation is inversely proportional to the rotational correlation time, so that it becomes a highly efficient transfer mechanism for solution NMR with very high molecular weights. As a first implementation, [15N,1H]-correlation experiments were designed that make use of cross-correlation between dipole-dipole coupling and chemical shift anisotropy of the 15N-1H-moieties for both CRINEPT and TROSY. When compared with INEPT-based [15N,1H]-TROSY these new experiments yielded up to three-fold signal enhancement for amide groups of a 110000 MW protein in aqueous solution at 4xc2x0 C., which has a rotational correlation time of about 70 ns. CRINEPT opens new avenues for solution NMR with supramolecular structures such as, for example, membrane proteins solubilized in micelles or lipid vesicles, proteins attached to nucleic acid fragments, or oligomeric proteins.
Structure determination of proteins by NMR in solution (Wuithrich, 1986) has so far been limited to the molecular weight range up to approximately 30 kDa (Clore and Gronenborn, 1997), and experiments yielding backbone assignments for 2H-labeled proteins up to about 60 kDa have been reported (Shan et al, 1996). In larger molecular species the standard experimental techniques (Bax and Grzesiek, 1993; Wider, 1998) lead to severe sensitivity loss due to transverse relaxation even when optimal isotope-labeling is used (LeMaster, 1994). The situation has recently been improved with the introduction of the TROSY technique, which reduces transverse relaxation during evolution and observation periods in heteronuclear NMR experiments (Pervushin et al., 1997; Salzmann et al., 1998). For example, for 15N-1H-moieties in proteins, significant reduction of transverse relaxation during the 15N evolution and amide proton acquisition periods can be achieved at the highest presently available 1H frequencies, and nearly complete cancellation is expected at 1H frequencies near 1 GHz (Pervushin et al., 1997; Wxc3xcthrich, 1998). TROSY has already been applied for detailed NMR studies of a protein with molecular weight above 100 kDa (Salzmann et al., 1998). Theoretical considerations indicate that TROSY will reach its limits at somewhat larger sizes because of rapid transverse relaxation during the INEPT-type (Morris and Freeman, 1979) magnetization transfers via scalar couplings between the different nuclei.
This invention presents a novel transfer technique for work with very large molecules, CRINEPT (Cross RelaxatIoN-Enhanced Polarization Transfer), which is based on cross-correlated relaxation (Goldman, 1984; Wimperis and Bodenhausen, 1989; Boyd et al., 1990; Brxc3xcschweiler and Ernst, 1991) and scalar couplings. The performance of cross-correlated relaxation for polarization transfer, CRIPT (cross-correlated relaxation-induced polarization transfer), is investigated for 15N-1N-moieties using a novel experimental implementation for very large particles in solution, where one has cross correlation between relaxation by dipole-dipole coupling (DD) and by chemical shift anisotropy (CSA). We then describe an initial implementation of CRINEPT in [15N,1H]-correlation experiments that make use also of TROSY during 15N evolution and 1HN acquisition periods. Data sets are recorded for a 2H,15N-labeled protein with an effective rotational correlation time of about 70 ns.
Theoretical Considerations
Transfer of in-phase 1H coherence to anti-phase 15N coherence in 15N-1H-moieties by cross-correlation between DD and CSA relaxation (Goldman, 1984; Wimperis and Bodenhausen, 1989; Brxc3xcschweiler and Ernst, 1991; Pervushin et al., 1997) is considered. Using the product operator formalism (Sxc3x8rensen et al., 1983) the spin operator I corresponds to 1H, S to 15N, and the two spins have a scalar coupling constant JIS and resonance frequencies (xcfx89I and xcfx89S. For large molecular sizes at high magnetic fields only terms proportional to the spectral density function at zero frequency, J(0), need to be considered (Pervushin et al, 1997). For isotropic rotational tumbling, J(0) is equal to 2xcfx84c/5, where xcfx84c is the isotropic rotational correlation time of the molecule. The evolution of in-phase coherence is coupled to the anti-phase coherence via the cross-correlation relaxation rate RC. Starting with in-phase magnetization on spin I at the start of the magnetization transfer period T,  less than Ix greater than (0), the build-up of anti-phase coherence in CRIPT during T can be described by                                                         ⟨                              2                ⁢                                  I                  x                                ⁢                                  S                  z                                            ⟩                        ⁢                          (              T              )                                =                                    sinh              ⁡                              (                                                      R                    C                                    ⁢                  T                                )                                      ⁢                          exp              ⁡                              (                                                      -                                          R                      I                                                        ⁢                  T                                )                                      ⁢                          ⟨                              I                x                            ⟩                        ⁢                          (              0              )                                      ,                  xe2x80x83                ⁢        with                            (        1        )                                          R          I                =                                                            2                5                            ⁡                              [                                                                            2                      9                                        ⁢                                                                  (                                                                              γ                            I                                                    ⁢                                                      B                            0                                                    ⁢                                                      Δσ                            I                                                                          )                                            2                                                        +                                                            1                      2                                        ⁢                                                                  (                                                                              ℏγ                            I                                                    ⁢                                                                                    γ                              S                                                        /                                                          r                              IS                              3                                                                                                      )                                            2                                                                      ]                                      ⁢                          τ              c                                +                      1                          2              ⁢                              T                                  1                  ⁢                  S                                                              +                                    1                              T                                  2                  ⁢                  I                                                      ⁢                          xe2x80x83                        ⁢            and                                              (        2        )                                                      R            C                    =                                    4              15                        ⁢                                          (                                                      γ                    I                                    ⁢                                      B                    0                                    ⁢                                      Δσ                    I                                                  )                            2                        ⁢                          (                                                ℏγ                  I                                ⁢                                                      γ                    S                                    /                                      r                    IS                    3                                                              )                        ⁢                          τ              c                                      ,                            (        3        )            
where rIS is the distance between the two nuclei involved, xcex94"sgr"I the CSA tensor of nucleus I, B0 the static magnetic field, and xcex3I and xcex3S are the gyromagnetic ratios of I and S. T2I and TIS account for the transverse relaxation of spin I and the longitudinal relaxation time of spin S.
The relative efficiencies of polarization transfer with CRIPT (Eqs. (1)-(3)) or with INEPT (Morris and Freeman, 1979) at variable rotational correlation times xcfx84c are compared in FIG. 1a. The build-up of anti-phase magnetization in INEPT is described by
 less than 2IySz greater than (T)=sin(xcfx80JIST)exp(xe2x88x92RIT) less than Ix greater than (0).xe2x80x83xe2x80x83(4)
Since in Eq. (1) the transfer time T appears always in a product with xcfx84c, the optimal transfer period for CRIPT is inversely proportional to xcfx84c. Therefore, with proper adjustment of T the maximal amount of magnetization that can be transferred by CRIPT is independent of the molecular size (FIG. 1a), where one has to consider, however, that the optimal T for xcfx84c values shorter than about 20 ns would be unreasonably long. In contrast, the efficiency of INEPT falls off rapidly with increasing size (Eq.(4), FIG. 1a).
The magnetic field dependence of CRIPT for a 15N-1H-moiety located in a xcex2-sheet of a fully 15N, 2H-labeled protein (FIG. 1b) shows that the maximum theoretical magnetization transfer with CRIPT is about half of the maximum transfer by INEPT, and that maximal CRIPT transfer for a 15N-1H moiety is obtained at about 1 GHz (Pervushin et al., 1997; Wxc3xcthrich, 1998; Salzmann et al., 1998). The FIGS. 1a and 1b further show that CRIPT becomes more efficient than INEPT for molecules with xcfx84c≳100 ns, but that INEPT contributes significantly to the polarization transfer up to xcfx84c≈300 ns.
Based on the observations on CRIPT and INEPT in FIGS. 1a and 1b, and considering that systems with xcfx84c values in the range 50-300 ns will be of special practical interest, we combined the two polarization transfer mechanisms in CRINEPT, where proton anti-phase coherence is generated during a delay T devoid of radio-frequency pulses, which results in the terms (5) and (6) for the CRINEPT transfer (obtained from the differential equation (32) in Goldman, 1984):
 less than 2IxSz greater than (T)=A1I less than Ix greater than (0)=cos(xcfx80JIST)sin h(RCT)exp(xe2x88x92RIT) less than Ix greater than (0)xe2x80x83xe2x80x83(5)
 less than 2IySz greater than (T)=A2I less than Ix greater than (0)=sin(xcfx80JIST)cos h(RCT)exp(xe2x88x92RIT) less than Ix greater than (0)xe2x80x83xe2x80x83(6)
Eqs. (5) and (6) are the x- and y-components of the resulting anti-phase magnetization, respectively. The relative orientation of the resulting total magnetization therefore depends on xcfx84c and the mixing time T, and the transfer efficiency of CRINEPT represented by the signal amplitude AI (see FIG. 1a) is proportional to the absolute value of the total anti-phase magnetization:
AI={square root over (A21I+A2I2+L )}={square root over (sinh+L (RCT+L )2+L +sin(xcfx80JIST+L )2+L )} exp(xe2x88x92RIT)xe2x80x83xe2x80x83(7)
With Eq. (7) the relative contributions of INEPT and CRIPT to the total polarization transfer can readily be evaluated, whereas Eqs. (5) and (6) contain a mix of polarization transfer via scalar coupling (trigonometric functions) and CRIPT (hyperbolic functions) in both terms. For short xcfx84c, the rate RC is negligibly small and only INEPT contributes to CRINEPT, whereas for long xcfx84c, RC becomes large and CRIPT is the dominant polarization transfer mechanism (Eq. (3) and FIG. 1a). In principle, CRINEPT is always superior to INEPT or CRIPT (FIG. 1a). However, free proton chemical shift evolution during CRINEPT transfers (FIG. 2c, FIG. 3a and FIG. 3b) has to be handled by additional pulse sequence elements, which may somewhat reduce the overall sensitivity (see below).
Pulse Schemes for Comparative Studies of Magnetization Transfer by CRIPT, INEPT, and CRINEPT
The three experimental schemes in FIGS. 2a-2c were used for measurements of the efficiency of a single transfer from in-phase magnetization on 1HN to anti-phase magnetization on 15N (arrows in FIGS. 2a-2c) by the three transfer types considered in the preceding section. In each of the experiments the 15N anti-phase coherence is frequency labelled during t1 and transferred identically to 1HN anti-phase magnetization with the two 90xc2x0 pulses on I and S. In all experiments the water magnetization is kept along the z-axis during the whole sequence, using water-selective pulses.
In the novel scheme used for CRIPT (FIG. 2a) the in-phase 1HN magnetization generated by the first 90xc2x0 pulse is transferred to anti-phase magnetization by cross-correlated relaxation, according to Eq. (1). The proton chemical shift evolution is refocused by a 180xc2x0 pulse, which also decouples the protons from 15N. At the end of the period T, 90xc2x0 pulses on I and S generate the anti-phase coherence 2IzSy. The magnetization flow can be described in short notation as Iyxe2x86x922IySzxe2x86x922IzSy (FIG. 2a).
In the INEPT scheme (FIG. 2b) the flow of coherence is Iyxe2x86x922IxSzxe2x86x922IzSy.
In the CRINEPT transfer measurement (FIG. 2c) the absence of 180xc2x0 radio frequency pulses during T results in magnetization transfer by cross-correlated relaxation as well as by scalar coupling. In addition, 1H chemical shift evolution occurs during T. The resulting anti-phase coherence at time a can be represented by the density matrix
"sgr"(a)=xe2x88x922IxSz (cos(xcfx89IT)A2I+sin(xcfx89IT)A1I)+IySz(xe2x88x92sin(xcfx89IT)A2I cos(xcfx89IT)A1I),xe2x80x83xe2x80x83(8)
where A1I and A2I are given by Eqs. (5) and (6). The CRINEPT transfer efficiency can be measured with two experiments that use, respectively, x or -y phase for the second 90xc2x0 proton pulse (FIG. 2c). With phase -y, the first term of Eq. (8) is detected, with phase -x the second term.
[15N,1H]-correlation Experiments Using CRINEPT and TROSY
For practical applications of CRINEPT we introduced a gradient during the period T (FIG. 3a), which changes Eq. (8) to
"sgr"(a)=xe2x88x922IxSz(cos(xcex93+xcfx89IT)A2I+sin(xcex93+xcfx89IT)A1I)+2IySz(xe2x88x92sin(xcex93+xcfx89IT)A2I+cos(xcex93+xcfx89IT)A1I),xe2x80x83xe2x80x83(9)
The dephasing along the z-axis due to the gradient is indicated by xcex93=G1xcex3Hxcfx84z, where xcfx84 is the length of the pulsed field gradient, G1 its strength, and z describes the position of the observed spins along the z-axis. Direct use of the CRINEPT transfer element as shown in FIG. 2c would result in reduced sensitivity, since only half of the components of Eq. (9) can be recovered. In addition, a refocusing element has to be introduced in the pulse sequence to get a phase-sensitive [15N,1H]-correlation experiment, as is demonstrated in the [15N,1H]-CRINEPT-TROSY experiment of FIG. 3a. Alternatively, when omitting the second 90xc2x0 proton pulse (FIG. 2c), zero-and double-quantum coherences are generated and all the terms of Eq. (9) can be transferred and refocused as demonstrated in the [15N,1H]-CRINEPT-HMQC experiment of FIG. 3b. 
FIG. 3a shows a fully relaxation-compensated CRINEPT-correlation experiment. The following description retains only the magnetization components that lead to a detectable signal during the acquisition period. At time point a of the pulse scheme, after the first time period T, the magnetization is described by Eq. (9). Due to the subsequent pulses only the first term of Eq. (9) is transferred to transverse magnetization on 15N, which is subsequently frequency-labeled during the time t1, yielding the following terms at time b:
"sgr"(b)=(2IzSy cos(xcfx89St1)cos(xcfx80JISt1)+Sxcos(xcfx89St1)sin(xcfx80JISt1))xc2x7(A2I cos(xcex93+xcfx89IT)+A1I sin(xcex93+xcfx89IT))xe2x80x83xe2x80x83(10)
The CRINEPT-based sequence elements between time points b and d refocus the precession of the proton chemical shift during the first CRINEPT element as well as the effect of the first gradient, and immediately before the last 90xc2x0 pulse on 15N the in-phase term of Eq. (10) is transferred to the following coherences:
"sgr"(c)=2IySz cos(xcfx89St1)sin(xcfx80JISt1)(A2I+A1I)A2SAIxSx+2IxSz cos(xcfx89St1)sin(xcfx80JISt1)(A2I+A1I)A1SAIxSxxe2x80x83xe2x80x83(11)
where AIxSx accounts for the relaxation of IxSxxc2x7A2S and A1S are calculated with Eqs. (5) and (6) after exchange of the indices I and S, using the relaxation rates RI and RC (Eqs. (2) and (3)). Finally, applying the last 90xc2x0 pulse on 15N with the phase xcexa82=x+arc tan(A2S/A1S) the following proton anti-phase coherence is generated (AS is calculated with Eq. (7) by replacement of the indices I with S):
"sgr"(d)=2IxSz cos(xcfx89St1)sin(xcfx80JISt1)(A2I+A1I)ASAIxSxxe2x80x83xe2x80x83(12)
The anti-phase term of Eq. (10) is transformed to in-phase:
"sgr"(d)=Iy cos(xcfx89St1)cos(xcfx80JISt1)(A2I+A1I)AIAIzSzxe2x80x83xe2x80x83(13)
where AIzSz accounts for the reduction of the signal amplitude by relaxation of the SzIz state during the first period T within the refocusing element of FIG. 3a. The in-phase and anti-phase components in Eqs. (12) and (13) are separated by recording two FIDs with inverted phase xcexa82 (FIG. 3a).
In the [15N,1H]-CRINEPT-HMQC experiment of FIG. 3b both transfer elements are based on CRINEPT and the different phases and chemical shift modulations obtained with the transfer method of Eq. (9) are optimally refocused by the 180xc2x0 pulse on protons and the second CRINEPT element. The experiment is based on [15N,1H]-HMQC (Mxc3xcller, 1979), which does not contain TROSY compensation during the 15N evolution period but benefits from the absence of DD relaxation during the multiple quantum state.
Experimental
The NMR experiments were recorded with 7,8-dihydroneopterin Aldolase from Staphylococcus aureus. This protein is a homo-octamer with subunits of 121 amino acid residues. For the experiments in this paper it was uniformly isotope-labeled with 15N and in the extent of 75% with 2H, and it was studied at 4xc2x0 C. in H2O using a protein concentration of 0.4 mM. Based on T1 and T2 relaxation measurements of 15N (Kay et al., 1989) at 20xc2x0 C., the rotational correlation time xcfx84c under the conditions of the present experiments was estimated to be 70 ns. All NMR spectra were measured on a Bruker DRX-750 spectrometer equipped with four radio-frequency channels and a shielded pulsed field gradient along the z-direction.
Results
Magnetization transfer by CRIPT, INEPT and CRINEPT
To provide a foundation for the use of CRINEPT, we evaluated the optimal transfer times T for each of the three transfer mechanisms of FIGS. 2a-2c in a macromolecular system with an effective rotational correlation time of about 70 ns, using the 15N,2H-labeled S. aureus aldolase in H2O solution at 4xc2x0 C. The transfer efficiencies were measured from serial experiments using the pulse sequences in FIGS. 2a-2c with variable transfer times T. The build-up curve for CRIPT from 0 to 15 ms shows a fast increase followed by a plateau and an exponential decay, which corresponds to Eq. (1) as demonstrated by the close fit obtained with the simulation (FIG. 4). The optimal transfer delay is about 6 ms, with an observed range of 4 to 13 ms for different 15N-1H groups in the aldolase.
For INEPT transfer with the same 15N-1H-moiety as shown for CRIPT, the optimal delay is about 3 ms (FIG. 4), and only about 50% of the maximal transfer that would be obtainable with-out relaxation is achieved (FIGS. 1a and 4)). Nonetheless, for the presently studied system with xcfx84c=70 ns the observed INEPT transfer maximum exceeds the maximal CRIPT transfer about 1.5-fold (FIG. 4).
The experiments with CRINEPT (FIG. 4) and the fitting of Eq. (9) to the measured build-up confirm the theoretical prediction (FIG. 1a) that CRINEPT is superior to INEPT and CRIPT for all correlation times, provided that all coherences of Eq. (9) can be used further in the experiment. In the system of FIG. 4 the optimal transfer delay for CRINEPT is around 4 ms and thus lies between the optimal T values for the two basic experiments. The relative maximal transfers for CRINEPT, INEPT and CRIPT are about 7.6:5:3.4 (FIG. 4), which coincides well with the theoretical values for a protein with xcfx84c=70 ns (FIG. 1a).
[15N,1H]-correlation Experiments with CRINEPT Transfers
The [15N,1H]-CRINEPT-TROSY experiment (FIG. 3a) includes transverse relaxation-optimization during the transfer delays as well as the evolution periods. Measurements for the peak shown in FIG. 5 gave a two-fold signal increase when compared to [15N,1H]-TROSY. For other fast relaxing aldolase signals, gains between 1.5 and 3 were obtained. The experimental scheme of FIG. 3a has been designed to select the two multiplet components of the 15N-1H signal indicated by the filled circles in FIG. 5 (Eqs. (12) and ( 13)). In the spectra obtained with the aldolase the more rapidly relaxing one of these two components is in most signals broadened beyond detection (middle panel of FIG. 5a). Both components could be observed only for one highly flexible backbone 15N-1H-group and for some of the arginine side chains.
The [15N,1H]-CRINEPT-HMQC experiment (FIG. 3b) has no loss of magnetization due to proton chemical shift evolution during the CRINEPT element. Since there is no TROSY-compensation in the indirect dimension, there is an approximately two-fold increase of the 15N-line-width when compared with [15N,1H]-TROSY (FIG. 5). After application of a strong Gaussian window function in the 15N-dimension to enforce a comparable 15N-line-width, a 1.4-fold increase in signal-to-noise was measured when compared to standard [15N,1H]-TROSY (FIG. 5). Two multiplet peaks per 15N-1H-moiety are expected, since no decoupling is applied during acquisition (FIG. 3b). However, due to the fast relaxation of the lower-field component during the transfer elements and the t1-evolution, only the slowly relaxing component was detectable for most 15N-1H groups in the aldolase (FIG. 5b). This [15N,1H]-correlation peak is in a different position from those of any of the components of the 4-line signal, i.e., it is shifted about 45 Hz upfield along xcfx891(15N) when referenced to the TROSY-component (FIG. 5).
Discussion
Currently, nearly all heteronuclear multidimensional NMR experiments use INEPT to transfer magnetization between different spin species (Wider, 1998), but the efficiency of INEPT deteriorates with increasing rotational correlation time xcfx84c (FIG. 1a). In contrast, transfer of polarization by CRIPT is independent of xcfx84c. Further, in amide groups the efficiency of CRIPT increases with the strength of the external magnetic field up to about 1 GHz proton frequency, whereas the sensitivity of the INEPT transfer deteriorates further due to increased CSA relaxation. The present experiments with the 110 kDa S. aureus aldolase confirm the theoretical considerations of FIGS. 1a and 1b, showing that for molecules with rotational correlation times from approximately 50 to 300 ns both INEPT and CRIPT promote substantial polarization transfer between 1H and 15N in amide groups, which is exploited in CRINEPT.
The two CRINEPT-based [15N,1H]-correlation experiments presented in FIGS. 3a and 3b can be used as building blocks for a wide variety of NMR experiments with large molecules, which may include simple two-dimensional experiments (Bodenhausen and Ruben, 1980; Ernst et al., 1987, Wxc3xcthrich, 1986;), triple-resonance experiments for sequential and intraresidual backbone assignments (Montelione and Wagner, 1989; Ikura et al., 1990; Bax and Grzesiek, 1993), experiments for side chain assignments (Bax et al., 1990; Ikura et al., 1991; Grzesiek and Bax, 1992), as well as experiments for studies of molecular dynamics (Kay et al., 1989; Peng and Wagner, 1992; Kay et al., 1992; Dayie and Wagner, 1994). For example, the [15N,1H]-CRINEPT-TROSY experiment (FIG. 3a) might be used in triple-resonance experiments, and [15N,1H]-CRINEPT-HMQC (FIG. 3b) might be the preferred choice in multi-dimensional experiments with short 15N-evolution periods, such as 3D 15N-resolved [1H,1H]-NOESY.
For systems with xcfx84c≳300 ns, CRIPT is predicted to be nearly as sensitive as CRINEPT (FIG. 1a). CRIPT could then also be used as a xe2x80x9cfilterxe2x80x9d to eliminate resonances originating from smaller molecules, since the optimal transfer time T is inversely proportional to xcfx84c (Eq. (1)).
Although the present invention is focused on magnetization transfer by cross-correlated relaxation between DD coupling and CSA, more general use of the CRINEPT principle may cover other types of cross-correlated relaxation. Overall, CRINEPT combined with TROSY opens new avenues for NMR studies with molecular weights of several hundred kDa, such as membrane proteins solubilized in micelles or lipid vesicles, proteins attached to large nucleic acid fragments, or oligomeric proteins.